Summary/Abstract Indirect calorimetry is based on measurement of the exchange of respiratory gases. All aerobic energy-releasing re- actions in the body use oxygen; oxygen consumption, V_O2, is proportional to energy expenditure (EE). Research- grade indirect calorimetry systems measure V_O2, carbon dioxide production, V_CO2, and minute ventilation, V_E, breath by breath and then average results over multiple breaths to obtain accurate measures of cardiorespiratory function. The maximum attainable V_ O2 (i.e., V_ O2max) is the gold standard for assessing cardiorespiratory tness. In cardiac rehabilitation (CR), which is our focus herein, patients with coronary artery disease or chronic heart failure undergo supervised exercise training to improve heart health. Most CR is performed in outpatient settings, where the lack of equipment and specially-trained sta have precluded objective measurement of key values, such as peak oxygen uptake (V_O2peak), which is maximum V_O2 attained by the patient in a particular exercise stress test (which is not necessarily her/his maximum attainable V_ O2). Measurement of V_ O2peak is desired for tracking CR progress and for prescribing exercise intensities. Due to its nonavailability in most outpatient settings, suboptimal, semi-quantitative substitute measures (e.g., heart rate reserve, Borg ratings of perceived exertion, or 6-minute walk test) are often employed. Additionally, with measurement of V_ E, the e cacy of CR at improving measures of breathing e ciency (e.g., the ratios V_ E=V_ CO2 and V_ E=V_ O2) can be computed, which is especially important for patients with high initial values. Like V_ O2peak, these e ciency metrics are seldom available in CR practice due to equipment and personnel limitations. To obtain such measures, patients have to be scheduled for a cardiopulmonary exercise test (CPET), which is expensive and, beyond an initial test, generally not covered by insurance. The proposed Mobile Metabolic Measurement (M3) system will overcome these limitations and can be used during CR exercise sessions to track patient progress on a frequent basis. Objectively measured V_ O2peak values can be used (along with heart rate) to optimize exercise prescriptions to enable maximum safe progress. The M3 system will o er a huge advance in quantitative CR patient monitoring. The M3 system is a novel, cost-e ective, portable device for accurately and reliably measuring V_O2, V_CO2, V_E, V_E=V_CO2, V_E=V_O2, the respiratory exchange ratio (R=V_CO2=V_O2), and EE (e.g., METS or kilocalories). The proposed M3 system's \one-button (turnkey) calibration-free operation and order-of-magnitude lower price compared to existing cart or mobile indirect calorimetry systems will make it practical and a ordable for use by non-experts in outpatient, hospital, laboratory, eld, educational, and even home settings. In addition to CR, other areas that could bene t from the M3 system include assessing the resting metabolic rate (RMR) of acutely ill and critically ill hospital patients to guide enteral and parenteral nutrition, monitoring the ventilatory status of patients being weaned from mechanical ventilators, and guiding weight loss by quantifying the impact of dietary and exercise regimens. Metabolic measurement is presently infeasible in many such applications.